This context exists because the cart-page block isn't actually restricted to the cart page — it can be placed on the home page, a collection page, or search results, and it behaves identically wherever it lands, since it's the same theme app block reading the same cart and customer state regardless of which template it's dropped into.
The tradeoff is that it loses product-page's Trigger rules, since there's no single "product being viewed" on a collection or search page the way there is on a PDP. What it keeps is everything else: full behavior-mode range, carousels, and hex-color styling. In practice this context is most useful for cart-value-driven messaging (progress bars, cross-sell prompts) that makes sense regardless of what page a shopper happens to be browsing.